a) Field of the Invention
The present invention concerns the preparation of dispersions to produce films of positive electrodes for use in polymer electrolyte batteries of the lithium polymer type (LPB), by pre-dispersion or co-grinding of the solid powders. More particularly, the present invention concerns a process for the preparation of polymer dispersions with a high content of solid particles and with at least one of the types of particles consisting of materials with a high specific surface (&gt;50 m.sup.2 /g) or oil absorption capacity, which dispersions still maintain rheological properties adequate for films with relatively low porosities and a few defects.
b) Description of Prior Art
Lithium/polymer electrolyte batteries of the LPB type are manufactured by superposing three main types of films: a film of lithium, a film of an electrolyte comprising a polymer and a salt of lithium, and a film of a positive electrode. Each of these films has a thickness between 5 and 50 .mu.m, for a total thickness of 100 to 150 .mu.m for the elementary film of battery. About thirty meters of elementary film, 15 cm wide, are typically required to give a battery of 100 Wh.
The films of positive electrode are prepared by coating, on an aluminum foil or metallized plastic film, used as an electrical collector, a dispersion containing an electrochemically active material such as vanadium oxide, some carbon black to ensure electronic conduction, a polymer-salt electrolyte to ensure ionic conduction and the mechanical bond between the solid particles mentioned above, and appropriate solvent mixtures, based for example on acetonitrile.
The amounts of active material and carbon black should be high, so as to ensure a good contact between the particles in the dried films of positive electrode, a good electrochemical performance during the cycles of charge and discharge of the battery, and the highest energy content possible, in terms of volume as well as of mass. In practice, however, there are limits to the quantities which may be incorporated, over which either the dispersion are too viscous to be coated in the form of thin films, or the films have lost all their mechanical cohesion, or still the gains in volume are more than offset by the losses resulting from the increasing porosity which appears in the films.
This is particularly true in the presence of materials with very high specific surfaces (&gt;50 m.sup.2 /g) such as carbon black, which requires more liquid and binding agent to be suitably wetted. Thus, the porosity of films containing volume percentages in active material/carbon black/electrolyte higher than 25/6/69 in the dried films, already start increasing rapidly, while the desirable volume percentages for good electrochemical performances should rather stand from 30/8/62 to 40/10/50.